The present invention relates to a method for determining the remaining service life of the switching contacts in an electric switching device and to an electric switching device including an evaluation unit that can be used in determining the remaining service life of the switching contacts in the device.
German Patent Application No. DE 40 28 721 C2 describes a method for determining the remaining service life of switching devices and an arrangement for carrying out the method. According to this method, only measurable values are used which can be measured in the cables of the device. Here, the structural changes to the switching device are to be intentionally avoided.
An object of the present invention is to provide a method and a device for carrying out the method by which the characterization of switching contact states with respect to the expected service life, or with respect to the respective existing state of an electric switching contact is improved.
The present invention provides a method for determining a remaining service life of switching contacts in an electrical switching device. The method includes operating the switching contacts so as to produce structure-borne sound signals, capturing the structure-borne sound signals, performing a Fourier transform on the structure-borne sound signals in a plurality of discrete time windows so as to produce a sonogram, determining a plurality of function values using the sonogram, evaluating the plurality of function values using an evaluation device.
The invention further provides an electrical switching device the includes a first switching contact, a second switching in an operable relationship to the first switching contact, a sound sensor configured to capture structure-borne sound signals produced by an operation of the first and second switching contacts, and an evaluation device configured to evaluate the structure-borne signals so as to determine a remaining service life of at least one of the first and second switching contacts.
According to the present invention, structure-borne sound signals of the switching contact arrangement which are produced by the switching operation in the contact pieces are captured, the captured structure-borne sound signals are subjected to a Fourier transform in discrete time windows to produce a sonogram, and individual function values are selectively determined from the sonogram and evaluated using an evaluation device. Preferred function values are the peak value of at least one sonogram amplitude, the rise time of a sonogram amplitude from the occurrence of the signal to its maximum value, the decay time of the sonogram amplitude as well as the centroid of the amplitude area or of the amplitude space of a sonogram amplitude or of an amplitude space.
In a preferred embodiment, by selectively choosing three different frequency bands (frequency ranges) within which to capture and evaluate structure-borne sound signals of the contact pieces, interference effects are suppressed to the greatest possible extent (frequency ranges having high noise components are selectively masked) and meaningful signals are individually determined for each switching contact. According to the present invention, this is achieved in that an integral is formed over the signals of the structure-borne sound waves of a switching contact in at least two of three frequency bands, that in the third frequency band, either an integral of the occurring sound signals is formed as well or only the amplitude value of the sonographically determined sound signal shape (sonic response) is determined, and the values obtained in this manner from the three frequency bands are evaluated using an evaluation device. In this manner, the structure-borne sound signals occurring inside a switching contact during a switching operation (opening and/or closing operation of the switching contacts) are captured and evaluated to assess the remaining service life.
Since a contact piece has particularly characteristic properties in different frequency bands which properties change as a function of its state of wear, it is possible to make a precise statement on the wear state and the associated remaining service life of the switching contact by evaluating these properties.
The bandwidths of 20 kHz-35 kHz, 50 kHz-125 kHz and 175 kHz-225 kHz have been found to be particularly preferable frequency ranges for the three frequency bands. In this connection, within the first frequency band (20-35 kHz), the amplitude response of the sonic response is determined from an appertaining sonogram and, within the second and third frequency bands (50-125 kHz and 175-225 kHz), a formed integrated value of the structure-borne sound signal amplitudes is formed. In a different procedure, it is also possible to integrate the sound signal amplitudes in all three frequency bands, and to evaluate these values afterwards.
The determination of the function values of the sonic response determined in an appertaining sonogram is preferable performed in a time interval of three to six milliseconds after the closing of the switching contacts (or the occurrence of the first structure-borne sound signal).
The method according to the present is carried out using a switching device which features an evaluation device.
In a first embodiment, the evaluation device is integrated in the switching device in addition to with sensors for structure-borne sound. In this context, the structure-borne sound sensors are coupled to the switching contact in the form of piezoelectric elements in a manner which permits capture of sound waves arising in the switching contact or in the contact piece itself.
In a preferred embodiment, the structure-borne sound sensors are arranged on the contact piece carrier via a ceramic crystal carrier or via an element which is operationally connected to the contact piece carrier with respect to the transmission of the structure-borne sound waves. In this manner, a galvanic separation between the sound sensor and the contact piece carrier is achieved while at the same time ensuring high-quality transmission of the structure-borne sound waves.
In a second embodiment, the evaluation device is designed as a separate device or as a part of a multipurpose device (multipurpose measuring instrument or diagnostic unit). In this context, as described above, the sound sensors are arranged immediately in the switching unit itself and can be contacted from outside via an interface. Thus, the expected remaining service life of the switching contacts of a switching device according to the present invention can be determined, when necessary, via an external evaluation device.